Several members of the H,K-ATPase family of ion pumps participate in renal K transport. This class of P-type ATPases includes the gastric H,K-ATPase as well as a number of "non-gastric" H,K-ATPase isoforms. Physiologic studies suggest that these enzymes operate predominantly at the apical surfaces of tubule epithelial cells. While much has been learned about the patterns of K,K-ATPase isoform expression and its response to stress, the functional and cell biologic attributes of these pumps remain largely unelucidated. We have studied the properties which is responsible for the apical sorting of the gastric H,K-ATPase alpha-subunits, indicating that different mechanisms determine these molecules' polarized distributions. Our analysis of ion fluxes driven by a "non-gastric" H,K-ATPase isoform suggests that it exchanges Na (rather than H) for K under normal circumstances. Thus, the individual H, K-ATPase isoforms in situ are regulated by endocytosis, which is mediated by an endocytosis signal in the cytoplasmic tail of the gastric H,K-ATPase beta-subunit. Transgenic mice expressing a version of the protein in which the signal has been disabled exhibit constitutively active renal K resorption. The identities of the K,K-ATPase isoforms which are normally subject to endocytic regulation and the nature of the participating epithelial cell machinery have yet to be established. To further understand the processes which govern H,K-ATPase function in the kidney, we will: 1) identify the sorting signals which target these pumps to the appropriate surface domains of diverse renal epithelial cell types; 2) complete the functional characterization of a "non-gastric" H,K-ATPase in vitro and identify the individual transport processes driven by each pump isoform in situ and 3) examine the role of endocytosis in regulating renal H,K-ATPase activity in situ and establish the molecular interactions and cell biologic mechanisms through which a complex collection of ion pumps participate in the maintenance of systemic K balance.